First get someone to MEASURE air flow with a flow hood or other device,
NOT their hand. you should have around 400 CFM per ton. CFM=Cubic
Feet per Minute. Installing bigger ducts would help, but the entire
duct system needs increased, not just 2 fittings. A 90 degree elbow is
equal to from 30 to 80 feet of straight duct if it is rectangular duct
and 10 feet omly if it is round duct. note that to double air flow
with a larger fan, you have to have 4 times the duct static pressure
(square the increase) and 8 times the blower horsepower (cube the
increase in air flow). These are called the FAN LAWS. The only way
around that is a more EFFICIENT blower. but the best thing is to
measure the air flow first. Note also that you may want to install a
larger coil which would have a larger surface area and therefore a
lower pressure drop. But that may cause a humidity problem, depending
on where you live (climate).